1. Field of the Invention
The present invention relates to a developing device for use in an image forming apparatus such as a facsimile or copying apparatus, and more particularly to a developing device for performing an electrophotographic process in such an image forming apparatus.
2. Description of the Related Art
FIG. 12 shows the structure of an image forming apparatus including a conventional developing device. As shown in FIG. 12, a photosensitive drum 1 having an outer photosensitive surface is rotated in a clockwise direction by a rotary drive mechanism (not shown). As also shown in FIG. 12, a charging device 2, an exposure device 3, a developing device 4, a transfer device 6, and a cleaning device 8 are arranged about the outer surface of the photosensitive drum 1. The photosensitive drum 1, the charging device 2, the developing device 4, and the cleaning device 8 are integrally supported by a side cover 9 to form a process unit 100.
The developing device 4 comprises a toner hopper 41, a toner pack 42, a feed roller 43, a developing roller 44, a developing blade 45, a support rod 46, a leaf spring 47, a support 48, a reinforcing plate 49, a toner sensor 50, and an agitator 51.
The toner hopper 41 is a hollow container having an upper open portion for storing toner therein. The toner pack 42 is mounted on the upper open portion of the toner hopper 41. The toner pack 42 is filled with toner and its opening is sealed with a seal sheet (not shown). When the seal sheet is removed and the toner pack 42 is mounted on the toner hopper 41, the toner from the toner pack 42 empties into the toner hopper 41. The toner sensor 50 is provided at a bottom portion of the toner hopper to detect the presence of toner.
The feed roller 43 is arranged at an opening on a side surface of the toner hopper 41 such that it is partly located in the toner hopper 41. The feed roller 43 is driven by a motor (not shown). The developing roller 44 lightly contacts both the photosensitive drum 1 and the feed roller 43. The feed roller 43 and the developing roller 44 are rotated in a counterclockwise direction by a rotary drive mechanism (not shown). The feed roller 43 carries the toner stored in the toner hopper 41 and supplies it to the developing roller 44. The developing roller 44 carries the toner provided by the feed roller 43 and supplies it to the outer surface of the photosensitive drum 1.
The cylindrical support rod 46 is arranged parallel to and above the developing roller 44 to support the developing blade 45 in contact with the developing roller 44. The leaf spring 47 is fixed to the support 48 and urges the support rod 46 toward the developing roller 44 with a set force and thereby urges the developing blade 45 against the developing roller 44. The support 48 is fixed to the side wall of the toner hopper 41.
In the facsimile apparatus having the structure as described above, an image is printed in the following manner.
First, the charging device 2 charges the outer photosensitive surface of the photosensitive drum 1 to a predetermined potential (e.g., -600 V). Subsequently, the exposure device 3 exposes the charged photosensitive surface of the photosensitive drum 1 in accordance with an image to be printed, thereby forming an electrostatic latent image on the surface of the photosensitive drum 1. Then, the developing device 4 develops the electrostatic latent image formed on the photosensitive surface of the photosensitive drum 1, as follows.
The agitator 51 rotates and carries the toner in the toner hopper 41 to the feed roller 43. The feed roller 43 is supplied with a predetermined voltage more negative than that of the developing roller 44. As a result, the feed roller 43 carries the toner, which tends to be negatively charged, to the developing roller 44 and the developing roller 44 carries and conveys the toner to the photosensitive drum 1. In particular, when the developing roller 44 rotates, toner carried on the developing roller 44 receives friction between the developing roller 44 and the developing blade 45. As a result, the toner is charged by this friction. A urethane resin layer is provided on the surface of the developing roller 44 and is positively charged by friction with the toner. Further, the developing blade 45 is made of silicone which is positively charged by the friction with the toner. Accordingly, the toner is negatively charged due to the polarization of the resin layer of the developing roller 44, and the polarization of the developing blade 45.
A developing bias, for example -200 V, having the same polarity as that of the potential of the photosensitive drum 1, is applied to the developing roller 44. The toner selectively attaches to the photosensitive drum 1 due to the presence of the electric fields of the electrostatic latent image, the developing bias, and the toner charge. The toner does not attach to the non-exposed portions of the photosensitive drum 1 since the potential at those portions of the photosensitive drum 1 are more negative than that of the developing roller 44. The toner attaches to the exposed portions of the photosensitive drum 1, however, since the potential at those portions of the photosensitive drum 1 are less negative than that of the developing roller 44. In this manner, a toner image corresponding to the electrostatic latent image is formed on the surface of the photosensitive drum 1. This toner image is transferred to a printing sheet (not shown) by a positive voltage of the transfer device 6.
After the printing sheet is separated from the photosensitive surface of the photosensitive drum 1, toner which is not transferred to the sheet and which remains on the surface of the photosensitive drum 1 is removed by the cleaning device 8.
In the image forming apparatus of FIG. 12, toner feeding parameters, such as revolution speed of the feed roller 43, revolution speed of the agitator 51, the voltage difference between the feed roller 43 and the developing roller 44, are held constant. Further, as the toner in toner hopper 41 is consumed, the densities of images formed on sheets of paper change. FIG. 13 shows changes in image density in accordance with the remaining amount of the toner in the toner hopper 41. As shown in FIG. 13, as the amount of toner remaining in the toner hopper 41 decreases, image density also decreases. Thus, if the toner feeding parameters are set so that image density is appropriate when there is a lot of toner remaining in the toner hopper 41, image density is poor when there is little remaining toner in the toner hopper 41. As a result, it becomes difficult for the user of the apparatus to see the developed image formed on the paper when toner levels are low and the user must operate a key unit of the image forming apparatus in order to increase image density. Similarly, if the toner feeding parameters are set so that image density is appropriate when there is little toner left in the toner hopper 41, the image density is too great when there is a lot of toner in the toner hopper 41, e.g., when a new toner pack 42 is attached to the toner hopper. As a result, the user must operate the key unit of the image forming apparatus to decrease image density.
Further, as described above, the toner receives friction between the developing blade 45 and the developing roller 44. As a result, the toner is charged. However, as the amount of the toner in the toner hopper 41 is great, the weight of the toner is likewise great. When the toner with its own great weight is conveyed to the developing blade 45 by the feed roller 43 and the developing roller 44, the toner presses the developing blade 45 toward the photosensitive drum 1. Accordingly, the pressure of the developing blade 45 against the developing roller 44 decreases in this state. As a result, some of the toner tends to pass between the developing blade 45 and the developing roller 44. Therefore, some of the toner does not receive enough friction between the blade 45 and the roller 44. Accordingly, some of the toner is not sufficiently charged. This insufficiently charged toner is conveyed to a contact portion between the developing roller 44 and the photosensitive drum 1 and the roller 44 presses the toner to the drum 1. In this case, if the toner is not sufficiently charged, the electrostatic repulsive force acting on the insufficiently charged toner particles in a non-image portion of the latent image is smaller than the adhesive force which causes the toner particles to be transferred to the photosensitive drum 1. Therefore, the insufficiently charged toner is adhered not only to an image portion of the photosensitive drum 1 but also to a non-image portion thereof. The condition in which toner is adhered to the non-imaged portion of the photosensitive drum 1 is called fog.
A relationship between the amount of toner inside the toner hopper 41 and the amount of fog on the photosensitive drum is shown in FIG. 14. The degree of fog on the photosensitive drum 1 is expressed in a following manner. A first mending tape is adhered to the photosensitive drum 1 corresponding to the non-image background area. After that, in the event that the first mending tape is removed from the photosensitive drum 1 and adhered to a white paper, a first reflectance is sampled from the first mending tape. A second reflectance is sampled from a second mending tape which is not used and adhered to the white paper. The degree of fog on the photosensitive drum 1 is expressed by a difference between the first reflectance and the second reflectance. As shown in FIG. 14, the greater the amount of toner inside the toner hopper 41, the greater the fog on the photosensitive drum 1. Further, at the transfer device 6, the toner adhered to the non-imaged portion is transferred to the portion of the paper corresponding to the non-imaged portion at the transfer device 6.
FIG. 15 shows a relationship between the number of recorded sheets of paper on which toner has been supplied by the toner hopper 41 and the amount of fog on the photosensitive drum 1. As shown in FIG. 15, the smaller the number of recorded sheets, the greater the fog. Thus, the amount of fog is great immediately after the toner is supplied with the toner hopper 41.
If the feeding parameters are set so that the fog is appropriate when there is a lot of toner remaining in the toner hopper 41, the image density is poor when there is little remaining toner in the toner hopper 41. Similarly, if the toner feeding parameters are set so that the image density is appropriate when there is little toner left in the toner hopper 41, the fog is great when there is a lot of toner in the toner hopper 41.